The present invention relates to a method of producing sintered friction laminae for clutches and brakes by scattering metal powder over a steel substrate and sintering the powder in a sintering furnace in a protective gas atmosphere containing nitrogen, in particular split ammonia gas.
In multiple disc clutches and brakes, sintered friction laminae are used which consist of steel substrates which are provided on one or both sides with a friction coating sintered thereon and which comprise driver elements, e.g., teeth for the transmission of power, on at least one of their edges, on the inside, and/or the outside. The efficiency of such clutches and brakes is determined to a large extent by the durability of the steel substrates and in particular the driver elements thereof. However, even with regard to the dimensions and the weight of the clutches and brakes, it is impossible to employ steel substrates of excessive dimensions. Therefore fatigue fractures of the friction laminae occur in particular in the clutches and brakes of heavy duty drives and reversing drives. Moreover, wear effects occur at the driver elements, which effects are caused by the impact between the co-operating driver elements of the friction laminae and the receiving flanks of the engaging bodies.
In order to provide a remedy, endeavours have been made to increase the resistance of the steel substrates and to render them more resistant in particular in the critical zones. The increase of the life by the use of steels of higher quality or by hardening the steel substrates by means of a heat treatment is, however, not possible without difficulties. During the heating in the sintering furnace when the friction coatings are sintered upon the substrates, the latter are also heated and annealed to a greater or lesser extent. Therefore, hardening the steel substrates prior to sintering is only partly effective. Belated hardening after the deposition of the sintered friction coatings is opposed by the fact that the friction coatings are influenced disadvantageously by such a heat treatment, in particular when friction coating components are used the melting temperatures of which are lower than the hardening temperature. Even the induction hardening sometimes used for the steel substrates in the region of the teeth does not provide any substantial increase of the life resistance, although it does lead to an increase of the wear resistance in the region of the flanks of the teeth.
In more recent times, the application of the thermochemical diffusion treatment has become known for increasing the life and the wear resistance of steel components. In this case a diffusion of various elements, such as nitrogen, carbon, boron and chromium is enforced in particular at the critically loaded locations of the steel components. In the case of sintered fricton laminae an improvement of the edge layer properties of the steel support members in conjunction with an increase of the life could be attained by a nitrogenizing treatment. For this purpose, however, a proper heat treatment process of the finished sintered friction laminae is necessary which renders the production method considerably more expensive and is therefore used only in critical load cases.
Finally, it is known from German Pat. No. 950,298 to carbonize clutch laminae with a coating layer sintered thereon, at certain locations which are to have an increased rigidity; the carbonizing being performed simultaneously with the sintering. After the steel substrates have been provided with the friction coatings, the deposited coating is removed from the steel substrates at the locations to be carbonised, i.e., in the region of the teeth. Thereupon the laminae are stacked one upon the other at the locations to be carbonized with the interposition of a graphite or carbon disc or any other carbon delivering insert means, and are subjected to a heat treatment. This method is complicated and not suitable for modern mass production for cost reasons. Moreover, carbonizing is effected at temperatures around 900.degree. C., whereas for pressure sintering mainly used for sintered friction laminae produced by the scatter method, the temperatures lie in a considerably lower region. Moreover, the rigidity properties are not improved by a sufficiently great amount even by this carbonizing, in particular since effective hardening by carbonizing is possible only in conjunction with subsequent quenching baths which, however, cannot be used on account of the sintered friction coating.
The object of the invention is to improve the production method referred to above, of sintered friction laminae, in such a manner that a thermochemical diffusion treatment of the steel substrates and thus an increase of the life of the friction laminae is attained without an increase worth mentioning of the cost of the production of, and without a disadvantageous effect on, the friction coatings.